The present invention relates to flow imaging, in particular to suppression of echoes from stationary or slow moving reflections in an image.
Flow of media through a conduit, such as fluid through a pipe or blood through a blood vessel for example, may be imaged by transmitting waves such as ultrasound waves into the conduit and detecting echoes reflected from the flowing media and the surrounding structure. However, a problem with such imaging is that echoes from stationary or slow moving reflecting acoustic boundaries in the system such as pipe walls, blood vessels, tissue structures etc are far stronger than those from the flowing or time varying media such as water, oil, blood etc. The stationary or slow moving echoes may for example be 100 times larger than the echoes from the flowing media such that it is very difficult to precisely determine data for the flowing media.
In order to more precisely determine the data for the flowing media, the signal for the static or slow moving reflections is generally cancelled in the digital domain by digitally subtracting the static or slow moving element from the digitized incoming signal, as shown for example in EP 0 947 853. However, because of the very large difference between the magnitude of the signal from the static or slow moving reflections and the signal for the flowing media, for example 100:1, only a very small proportion of the resolution of the analogue-to-digital converter is used to digitize the portion of the incoming signal relating to the flowing media of interest, for example 1%. Furthermore, the extent to which the combined analogue signal can be amplified prior to analogue-to-digital conversion is limited by the need not to clip the static element, which has the flowing data of interest superimposed upon it, so that data is not lost. A high resolution analogue-to-digital converter is sometimes used but this can be very expensive often comprises the sampling rate possible.
It would be desirable to be able to reduce or remove the portion of a signal relating to echoes from static or slow moving reflecting acoustic boundaries in an image whilst overcoming or reducing one or more of the problems explained above.